1. Field of the Invention
The present invention relates to an image-blur preventing apparatus used in an optical instrument such as a silver-salt camera or a video camera.
2. Description of the Related Art
First described briefly with reference to FIG. 4 is the conventional technology related to the image-blur preventing apparatus to which the present invention is directed.
FIG. 4 is a block diagram to show the structure of a conventional image-blur preventing apparatus, in which reference numeral 1 designates a vibration gyro for detecting a shake of the apparatus as an angular velocity and outputting an angular velocity signal 2 as a detection output, and 3 an integration circuit for integrating the angular velocity signal 2 and outputting the integration result as an angular displacement signal 4 corresponding to an angle of deflection of the apparatus.
Numeral 20 denotes a variable (apical) angle prism (hereinafter referred to as VAP), which changes its apical angle to deflect a passing beam and to change an incident angle thereof into a lens unit 21, whereby blur correction is effected.
Numeral 15 denotes an actuator for changing the apical angle of VAP 20, which is driven by a drive control signal 14 from a drive control circuit 13.
Numeral 18 represents an apical angle sensor for detecting a displacement in apical angle of VAP 20, which obtains VAP apical angle displacement information 17 for example by an optical detection method and outputs a VAP apical angle displacement signal 19.
Numeral 9 is an adder, which adds the above VAP apical angle displacement signal 19 and the angular displacement signal 4 in opposite polarities and outputs the calculation result as a differential signal 10.
Numeral 10 is an amplifier, which amplifies the differential signal 10 to output an amplified differential signal 12. The amplified differential signal 12 is input into the drive control circuit 13. The drive control circuit 13 outputs the drive control signal 14 based on the amplified differential signal, as described above, and controls the drive of actuator 15.
A closed loop is formed by the group of components including (in a clockwise direction as depicted in FIG. 4) the adder 9, to the apical angle sensor 18, and each component in between. The closed loop functions to perform a control to keep the differential signal 10 always zero. As a result, the VAP apical angle displacement signal 19 is kept to coincide with the angular displacement signal 4, whereby the VAP 20 is driven by the apical angle displacement corresponding to an amount of the thus detected shake.
A beam 222 changed in incident angle by the VAP 20 is focused on the surface of CCD 223 by the lens unit 21. The conventional CCD 23 outputs an image signal 24.
FIG. 5 is a drawing to illustrate the operation of the apparatus in the conventional example shown in FIG. 4, showing a case in which a hand shake or the like inclines the camera system including the image-blur preventing apparatus at an angle .theta. downward about the point C. Next described referring to FIG. 5 are problems in the conventional image-blur preventing apparatus.
When shake detecting means including the vibration gyro 1 detects that the system is inclined at an angle .theta. downward, a blur correction operation is carried out such that the apical angle of VAP 220 (which is shown in FIG. 5 and is located immediately before the lens 221 on the optical axis thereof as shown in FIG. 4) is adjusted to incline upward by 8 the optical axis which was inclined downward by 8 with respect to the optical axis before being inclined. Then the inclination of the optical axis of an incident beam is corrected, whereby the camera system maintains the angle before inclination. It should be noted, however that the correction is only the angle is corrected in the conventional example and that an incident beam into the lens 21 is changed from B.sub.1 to B.sub.2 in the conventional example. Supposing a subject is at a distance equivalent to infinity, the blur correction is perfect in this conventional apparatus, because beams from B.sub.1 and from B.sub.2 can be deemed as being emitted from a single point. However, supposing a subject is at a finite distance L, a corresponding point on the subject moves from P.sub.1 to P.sub.2, producing an error angle .alpha. to also be corrected. Therefore, the correction is not perfect in this case. This error angle .alpha. in FIG. 5 is caused by the occurrence a shift change in the arrangement in which the rotation center of the camera is at the point C, apart from the correction optical member including VAP 20.